Optical fiber sensors are finding increased applications in civil, industrial and military fields. These sensors offer a myriad of advantages over their pneumatic and electronic counterparts that include increased sensitivity, geometrical flexibility, miniaturization, immunity from electromagnetic interference and multiplexing capabilities. Fiber optic sensors can be in-line, such as modal sensors, or external to the fiber, like the extrinsic Fabry-Perot interferometric sensor. Due to the photorefractive effect, a number of novel, intrinsic refractive index grating-based sensors have been recently proposed and demonstrated. These types of sensor elements are fabricated by exposing an optical fiber to a pattern of light at a wavelength at which the fiber is photosensitive. The light pattern photoinduces a refractive index variation or grating with the same spatial profile as the light pattern. Such gratings can be classified as either long- or short-period gratings. In general, the periodicity of long-period refractive index gratings are much larger than the wavelength of the operating light source (50-1500 microns). This is in contrast to short-period gratings which have periods less than the wavelength of the operating light source. Photoinduced, short-period Bragg grating sensors with periods less than one micron (1 micron=10.sup.-6 meter) have been widely demonstrated as strain and temperature sensors. These photoinduced Bragg gratings reflect light at a wavelength which depends upon the environmental parameter being measured. The light reflected from the grating remains in the core of the fiber as a guided, backward propagating mode and travels back within the fiber where it can be monitored remotely. Hence, either the reflection or the transmission spectrum of the short-period Bragg grating sensor can be used to monitor shifts in the wavelength of the reflected light in response to environment-induced changes in the periodicity of the grating.
A number of patents have been issued for short-period Bragg grating-based sensors and sensor systems. Meltz et al. (U.S. Pat. No. 4,806,012) disclose a distributed, spatially-resolving optical fiber Bragg grating strain gauge. This sensor comprises an optical waveguide including a core for carrying light injected at selected wavelengths. It is impressed and reflected with one or more periodic phase gratings for modifying the reflection and transmission of injected light at the position of said grating in response to conditions of local physical or thermal strain. The core contains a series of variable spacing Bragg reflection gratings written, impressed or otherwise applied by application of a variable two-beam ultraviolet interference pattern. A broad band light source is focused through the lens onto the exposed end of core. A beam splitter serves to direct the return beam from the core toward a suitable readout or spectrometer for analysis. Alternatively, a transmitted beam passing out of the end of the core could be analyzed. Glen (U.S. Pat. No. 4,950,883) discloses a fiber optic sensor arrangement having reflective gratings responsive to particular wavelengths. A multitude of fiber optic sensors are implemented on a single continuous optical fiber. A technique is provided for multiplexing the output of such sensors and for subsequent demultiplexing and evaluation to obtain independent measurements of each sensor's response in such a manner that each sensor is sensitive to even subtle changes in the parameter being monitored at the respective location of the body at which the sensing portion of the sensor is situated. Additional Bragg grating sensor-based patents include "Optical Waveguide Embedded Transverse Spatial Mode Discrimination Filter" (U.S. Pat. No. 5,048,913) by Glenn et al., "Method for Impressing Gratings within Fiber Optics" (U.S. Pat. No. 4,725,110) by Glenn et al., and "Distributed Multiplexed Optical Fiber Bragg Grating Sensor Arrangement" (U.S. Pat. No. 4,996,419) by Morey.
The salient features of the Bragg grating sensor is its short refractive index grating period which converts light traveling in the forward-propagating, guided fundamental mode to the reverse-propagating, fundamental guided mode. The light being converted between the forward and reverse-propagating modes remains in the core of the optical fiber where its interaction with the surrounding environment is minimal. In part, due to this minimal interaction with the surrounding environment, the strain, temperature, and refractive index sensitivity of short-period Bragg gratings sensors is relatively low. In contrast to short period Bragg gratings, we disclose a long period grating which couples light traveling in the guided modes to non-guided modes. The light contained in these non-guided modes interact with surface defects on the optical waveguide and is rapidly attenuated. These modes are therefore referred to as lossy. Hence, the object of our invention is to use a long period grating to convert light traveling in the guided modes of the optical waveguide to the lossy non-guided modes of the optical waveguide at one or more wavelengths determined by one or more environmental parameters being measured. As a result of the waveguide properties of the optical waveguide cladding being extremely sensitive to changes in the environment, such long period gratings have been found to provide higher sensitivity to environmental parameters than their short period Bragg grating sensor counterparts.
Mode conversion using long-period refractive index gratings have been previously demonstrated. Hill et al. ("Efficient Mode Conversion in Telecommunication Fibre Using Externally Written Gratings", Electronics Letters, Vol. 26, No. 16, pp. 1270-1272, Aug. 2, 1990) describe a photoinduced long-period grating used to convert light traveling in the forward-propagating fundamental guided mode to the forward propagating LP.sub.11 guided mode of the fiber. Using mode filters, the authors were able to produce absorption dips in the transmission spectrum of the photoinduced grating at wavelengths at which the mode conversion occurred. Bilodeau et al. ("Efficient, Narrowband LP.sub.01 .revreaction.LP.sub.02 Mode Converters Fabricated in Photosensitive Fibre: Spectral Response", Electronics Letters, Vol. 27, No. 8, pp. 682-684, Jan. 29, 1991) expanded on this concept by fabricating gratings using internal and external writing techniques to convert light traveling in the forward propagating fundamental guided mode to the forward propagating third order LP.sub.02 guided mode of the fiber. As with Hill's long-period grating, mode filters were required to introduce loss dips in the grating transmission spectrum because the process of converting light from one guided mode of the fiber to another guided mode of the fiber does not inherently introduce loss dips in the transmission spectrum of the grating.
Vengsarkar (U.S. Pat. No. 5,430,817) discusses an optical fiber communication component consisting of a photoinduced long-period grating which converts light traveling in the forward-propagating guided mode of the fiber to forward-propagating, non-guided lossy cladding modes of the fiber. He proposes the device as a spectral shape shifter to flatten the gain spectrum of erbium-doped amplifiers, to perform spectral cleanup in wavelength-division multiplexed communication systems, and for stabilization of fiber lasers. The device is also proposed as an inexpensive wavelength shift detector for use with reflection-based optical fiber sensors, such as the photoinduced short-period Bragg grating sensor described earlier. In addition, Vengsarkar discloses an optical fiber sensing system having a source of optical energy, a length of optical fiber including a short period reflective sensing grating for reflecting light, a long period grating coupled to the fiber for receiving light reflected from the short period grating and a photodetector for detecting the intensity of light through the device.
An object of the present invention is to provide an optical waveguide sensor arrangement for sensing at least one physical parameter which does not require the use of a Bragg grating.
Another object of the invention is to provide an optical waveguide sensor arrangement which senses changes in at least one physical parameter which uses a long period grating to couple guided modes to lossy non-guided modes to produce a wavelength transmission spectrum functionally dependent on the physical parameter sensed.